Effect of chlorpyrifos on immune function in rats.

A commercial formulation of chlorpyrifos was evaluated for effects on selected immune system functions in male Fisher 344 rats. Chlorpyrifos in an olive oil vehicle was administered by oral gavage twice weekly for 28 d at a dose of 5.0 mg/kg for each treatment. Body weight and organ/body weight ratios were unaffected by the chlorpyrifos. In contrast, chlorpyrifos impaired T-lymphocyte blastogenesis induced by concanavalin A (P = 0.03) and phytohemagglutinin (P = 0.023), but did not alter B-lymphocyte blastogenesis induced by lipopolysaccharide/dextran (P = 0.082. Humoral immunity (anti-sheep red blood cell), a T-lymphocyte macrophage-dependent response, was also reduced (P = 0.019) when the antibody response was expressed/10(6) spleen cells, although the response expressed/spleen was unaffected (P = 0.32), reflecting increased lymphocyte production. The total splenic lymphocyte counts in the chlorpyrifos-treated rats increased by 91% (P < 0.0001), therefore reducing the antibody response when expressed/10(6) spleen cells. Chlorpyrifos had no effect on macrophage phagocytosis (P = 0.27), but increased the relative percentage expression of CD5+ (P = 0.028) and CD8+ (P = 0.003). The presence of normal antibody and phagocytic responses in association with reduced T-lymphocyte blastogenesis and enhanced expression of specific cell surface antigens indicated that chlorpyrifos induced immune alterations associated with lymphocyte subpopulations.